1. Field of the Invention
The present invention relates to a radio receiver that corrects a tuning frequency of an RF tuning circuit on the basis of a frequency of a desired station and characteristics of a tuning element in the RF tuning circuit.
2. Description of the Related Art
Radio receivers normally include a means for preventing tracking error, which is the deviation between a tuning frequency of an RF tuning circuit and a frequency of an RF signal to be converted to a desired IF signal at a mixer circuit. A means has been proposed for correcting the RF tuning frequency in such radio receivers while taking into consideration the characteristics of a varicap diode as well as a coil constituting the RF tuning circuit. Such a radio receiver may be configured as shown in FIG. 1, and the tuning frequency of the RF tuning circuit is corrected according to the flowchart of FIG. 2.
In FIG. 1, the received RF signal is amplified by an RF amplifier 1, then tuned by an RF tuning circuit 2. The tuned RF signal is converted in frequency at a mixer 3 with a local oscillation signal from a local oscillator 4 to an IF signal having, for example, a center frequency of 10.7 MHz. The IF signal is amplified by an IF amplifier 5, then searched for FM at an FM detector 6.
An operation to set the tuning frequency of the RF tuning circuit 2 will be described next with reference to the flowchart of FIG. 2. First, the frequency of a local oscillation signal is set by a local oscillation control signal "a" of a controller 7. The frequency of the local oscillation signal is set so that the IF signal corresponding to the frequency of an RF signal for a desired station becomes 10.7 MHz. To accomplish this, the received frequency of the desired station is calculated at the controller 7 on the basis of the local oscillation control signal "a" (Si). On the other hand, a memory 8 stores characteristics of a diode 2a. The tuning frequency of the RF tuning circuit 2 is determined by a coil L and the varicap diode 2a. Since the coil L is fixed, the tuning frequency varies according to the capacitance of the diode 2a. Furthermore, since the capacitance of the diode 2a changes with a tuning correction signal "b", the tuning frequency changes according to the tuning correction signal "b". As a result, the tuning correction signal "b" increases as the tuning frequency rises as shown by a broken line in FIG. 3. The memory 8 stores characteristic resulting data which is discrete and has narrower data intervals as the frequency rises, as shown by the circles in FIG. 3. Since the received frequency of the desired station calculated in step S1 is to become the tuning frequency, the controller 7 determines which region of the tuning frequency contains the received frequency of the desired station (S2). For example, if the received frequency of the desired station is between the tuning frequencies of A and B, a formula for a straight line connecting the corresponding data A and B is determined (S3), and the tuning correction signal "b" for the desired station is calculated by substituting the received frequency of the desired station into the straight line formula (S4). If the received frequency is not between A and B, a straight line formula is determined from data of both ends of that region (S5). The calculated tuning correction signal "b" is impressed onto the diode 2a, the capacitance of the diode 2a is set, and the tuning frequency of the RF tuning circuit 2 is tuned to the frequency of the desired station (S6). In this manner, the tuning frequency of the RF tuning circuit 2 matches the frequency of the desired station so that tracking error is prevented.
However, the coil L and the varicap diode 2a have positive temperature characteristics. Thus, as the temperature changes within the radio receiver, the capacitance of the diode 2a changes. Since the tuning frequency of the RF tuning circuit 2 is corrected by the fixed tuning correction signal "b", the tuning frequency of the RF tuning circuit 2 changes from the change in temperature.